Look at cex.io right now. You can buy 1 GH/s for 0.2125 BTC, but 1 GH/s will never mine more than 0.1850 BTC (assuming an average difficulty increase of 20%).
Even assuming that cex.io is legitimate, why would anyone pay 0.2125 BTC to mine 0.1850 BTC?
I don't think this business model will ever work simply because there is no reason for cex.io to mine bitcoins for someone else when they could make more money by mining for themselves.
I've also done some math when first saw this site, it doesn't seem to be a good deal...
There is a quick way to estimate what a hash rate is worth:
Use this formula: VALUE = H * 7040691 / D / R, where
H = your hash rate in GH/s
D = current difficulty
R = average increase in difficulty each period for the next several months
For the last 8 months, R has been about 0.30 (30%), but that can't continue much longer. I predict R to be about 0.20 (20%) over the next several months. That's how I get a value of 0.185 BTC per GH/s. Others believe it will be much higher and miners seem to think it will be a little lower.
This calculation also doesn't include fees and expenses.
Cool, thanks for the explanation and calculation. I may have missed this, but where does 7040691 come from?